I don't think you'd want to have the vent open and the fan running while you are moving.
Epsar of Michigan (in Romulus) also sells them. They will install them, too.
Get the one with the rain sensor. Worth every penny.
It has three speeds, and will draw roughly 1,2 or 4 amps depending on fan speed. Even a 1 amp draw overnight will kill a starter battery in a van, so you're much better off using an aux battery with an isolator.
It's a DC fan so it hooks directly to the battery, and as has already been suggested, never hook a DC appliance directly to a battery without an inline fuse. Ever.
If you are going to use an inverter, hook the inverter directly to the battery with a fuse placed inline at the battery, and the Fantastic Fan can be connected at the same terminals as the inverter. If you plan on using an inverter, calculate how many DC amps the appliances will draw from the battery, and for how many hours a day, add those to the amps the Fantastic Fan will draw, and size your batteries accordingly.
The Fantastic Fan won't cool the van like an air conditioner would, but it will move a lot of air in short order, removing any heat buildup in the van. The better insulated the van, the less heat will build up. If you idle, the fan will, absolutely, suck diesel exhaust in the back doors, through the air vents, pretty much everywhere, so be aware of that. But mainly, turn it on, crack a window on the shade side of the van, and it will not only cool things down quote a bit, the biggest benefit is the influx of fresh air it provides.